Traditional rotary press cutters for envelopes have used a solid rotary cutter comprising a rigid arcuate knife on which a cutter blade is formed, so that rotation of the knife with the cutter blade formed thereon against a rotary anvil with the web of paper material trapped therebetween causes cutting of an outline in the paper material to the shape of the cutter blade. Such conventional rotary press cutters are slow to set up, and the accurate preparation of the cutters and cutter blades is a difficult and skilled task.
More recent proposals have been for the use of thin flexible foil cutting dies of the kind that have been used on label-cutting machinery. Such a flexible cutting die is formed by photographically printing onto a metal foil a chemical resist to the shape of the cutting line, and then etching away the foil for part of its thickness to expose in relief a raised cutting blade edge to the shape of the cutting line. The foil is then bent into an arc by wrapping it around the outer periphery of a rotary punching roller of an envelope cutting machine, and is secured to that rotary punching roller. The outer periphery of the punching roller may be formed by a cylindrical outer sleeve or an arcuate outer sleeve segment of the rotary punching roller of the envelope cutting machine, and the rotary punching roller with the flexible foil cutting die mounted thereon together form the rotary press cutter of the machine.
A problem exists in establishing a satisfactory method of securing the flexible foil cutting die to the outer periphery of the punching roller rapidly and accurately and immovably.
One prior proposed mounting system for a flexible foil envelope cutting die for an envelope cutting machine is disclosed in European Patent Application No. EP 312422A. The mounting system uses a rotary punching roller which has a cylindrical outer sleeve formed from two 180.degree. half-sleeves bolted together. One of the 180.degree. half-sleeves carries, recessed therein, an array of permanent magnets; and the flexible foil cutting die is attached to the respective 180.degree. half-sleeve by magnetic attraction only. One embodiment disclosed in EP 312422A comprises a locating ledge or abutment projecting radially outwardly from the periphery of one of the 180.degree. half-sleeves for locating a trailing edge of the flexible foil cutting die, but that is a locating abutment only and the teaching of EP 312422A is that the sole means of securing the flexible foil cutting die to the rotary punching roller is the attractive force of the magnets in the relevant 180.degree. half-sleeve. This teaching is drawn in its entirety from known label-cutting machinery which uses magnets to secure flexible foil cutting dies. The magnets used in such label-cutting machinery are adequate for the task because the paper material used for self-adhesive labels is generally softer and less abrasive on the cutter blades than that used for envelopes; and on a label-cutting machine the cutter blade cuts only through half the thickness of the laminated web of label, adhesive and backing paper, so that the impact on the blades as the leading edge of the blade catches the paper web of the label sheet in the nip with the anvil is considerably less on a label-cutting machine than on an envelope machine. However tests have shown that the same magnets are not adequate for envelope cutting machines. Over time, in an envelope cutting machine the continuous pounding of the blades against the rotary anvil as in EP 312422A would cause deterioration of the magnetic strength of the isotropic magnets used, and would also cause the flexible foil cutting die to creep over the magnet surface. EP 312422A has therefore been found to be totally inadequate to meet the very high standards of accuracy required in envelope manufacture.
It will be apparent from the above discussion that the envelope cutting and label cutting arts are in fact different arts. The skilled man would know that the vastly different impact forces on the cutter blades in those two arts require mutually significantly different mounting systems to mount the cutting dies on their rotary punching rollers.
An alternative proposal for the anchorage of a flexible foil envelope cutting die on a rotary punching roller is contained in British Patent Application No. GB 2133734A. The leading and trailing edges of a flexible foil cutting die are, according to GB 2133734A, creased back on themselves to form inturned leading and trailing edge flanges of the foil, and the resulting rearwardly inclined front flange and forwardly inclined rear flange are inserted in suitably formed and angled slots in the rotary punching roller. The front edge flange is also clamped in its slot.
Disadvantages of the above proposed mounting system are that the flexible foil of the cutting die loses so much structural strength when creased that the foil tends to split or tear in use along the acute angle of the front crease. This disadvantage is exacerbated by the fact that the flexible foil cutting die is mounted under tension as it is drawn around the rotary punching roller. That tension is necessary for the accurate register of the die on the roller. However thermal expansion of the flexible foil in use tends to cause it to slacken between its anchored ends so that accurate register is lost and metal fatigue in the foil, due to constant movement, becomes a real problem.